The present invention relates generally to an optical moisture sensor for mounting upon the interior surface of a windshield, and more particularly, to a compact optical moisture sensor having optical emitters, detectors, and optical components mounted on a planar circuit board which is positioned parallel to the interior surface. A coupler having collimator and focusing lenses is used to refract light beams as the light beams travel from the emitters, and is reflected from the outer surface of the windshield back to the detectors.
Motor vehicles have long been equipped with motor-driven windshield wipers for clearing moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance vision through the windshield. In most vehicles today, the windshield wiper system includes multi-position or variable speed switches which allow the driver to select a wide, if not an infinitely variable, range of speeds to suit conditions. Wiper controls are manually operated and typically include a delay feature whereby the wipers operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed which include a moisture sensor mounted on the windshield to automatically activate the motor when moisture is deposited upon the surface of the windshield or other vehicle window upon which a wiper may be employed, such as the rear window. By sensing rain or other moisture on the glass surface, the wipers can be controlled accordingly. Such wiper control systems free the driver from the inconvenience of frequently adjusting the wiper speed as the driving conditions change. Wiper control systems with optical moisture sensors have been incorporated into the production of several models of passenger cars. In order to increase the commercial use and consumer acceptance of the wiper control systems, there is a need for a more compact and less expensive optical moisture sensor.
Wiper control systems have employed a number of different technologies to sense the moisture conditions encountered by a vehicle, including conductive, capacitive, piezoelectric, and optical sensors. Optical sensors operate upon the principle that a light beam is diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. The systems which employ optical sensors have the singular advantage that the means of sensing disturbances in an optical path is directly related to the phenomena observed by the driver (i.e., disturbances in the optical path that affords the driver vision).
Noak (U.S. Pat. No. 4,355,271) discloses an optical moisture sensor having optical components mounted in a box-like housing attached to the interior surface of the windshield. The moisture sensor devices for controlling the windshield wipers of a vehicle as disclosed by McCumber et al. and Teder (U.S. Pat. Nos. 5,059,877 and 5,239,244) also disclose a box-like housing mounted upon the interior surface of the windshield for enclosing the optics and electronics.
In optical moisture sensors, light from an emitter is directed into the windshield at an angle of approximately forty-five degrees with respect to the windshield. The light is then reflected by the outer surface of the windshield at approximately a forty-five degree angle and is directed into a detector. The presence of moisture on the surface of the windshield affects the reflection of light at the air-glass interface at the outer surface of the windshield, and this change in reflected light is electronically processed and utilized as the signal for activating the windshield wipers. McCumber et al. (U.S. Pat. No.4,620,141) disclose an automatic control circuit for triggering a sweep of the wiper blades in response to the presence of water droplets on the exterior surface of a windshield.
When the angle of entry of the light beam into the windshield is greater than fifty degrees, a loss of signal frequently occurs. When the angle of entry is less than forty degrees, a loss of sensitivity occurs and the sensor is not able to properly detect moisture on the windshield. Consequently, it is essential that the angle of entry of the light beam from the emitter enter the windshield at approximately forty-five degrees.
The desired forty-five degree angle can be achieved by mounting the optoelectronic devices (emitters and detectors) at forty-five degree angles or by deflecting the light as it travels between the devices and the glass windshield. Stanton (U.S. Pat. No. 5,414,257) discloses an optical sensor having optoelectronic devices mounted on a circuit board at an appropriate angle with respect to the surface of the glass such that their optical axis' extend at the appropriate forty-five degree angle or can be deflected so as to do so. Stanton teaches devices cast from flexible epoxy resin and the bending of the leads of the devices at an angle to facilitate the angled mounting. The problem with bending the leads of the electronic devices is that most automated component insertion equipment cannot insert components with bent leads which increases the cost of assembling the circuit boards. In addition, the bent lead devices are less reliable from a performance standpoint.
The mounting of optoelectronic devices on circuit boards without bending the leads is disclosed in Zettler (U.S. Pat. No. 5,560,245). The emitters and detectors are mounted on small satellite circuit boards which are angled with respect to the main circuit board. The satellite circuit boards are angled to aligned the emitters and detectors at the appropriate forty-five degree angle with the windshield. Although this mounting configuration does not require lead forming, the use of such small circuit boards creates other problems. The small circuit boards used to mount the optoelectronic devices cannot accommodate the signal processing circuitry, which must be located on a separate circuit board. The use of multiple circuit boards and the orientation of the circuit boards in the housing of the sensor increases the size and cost of the sensor.
Conventional optoelectronic devices, including the new surface-mount technology devices (SMT's), are generally designed so that the optical axis is perpendicular to the circuit board on which they are mounted. Teder (U.S. Pat. No. 5661,303) discloses the use of a single circuit board mounted co-planar with the surface of the windshield which results in a low cost and compact sensor enclosure. However, this design requires optical components having optical axis which are approximately parallel to the optical axis of the optoelectronic devices. It is desirable to reduce the size and cost of the optical components to further reduce the size and cost of the moisture sensor.
Another way to reduce the size and cost of the optical sensor includes reducing the number of optoelectonic components. Noak discloses using a single detector to simultaneously detect two or more emitters. Muller (U.S. Pat. No. 5,015,931) discloses that several beams may be derived from a single nondirectional emitter. Such configurations provides the desired area of detection with a fewer number of detectors. McCumber et al. (U.S. Pat. No. 4,620,141) teach that a balanced configuration tends to reject the effect of ambient light. Emitters, however, typically vary by about 2:1 in signal strength. This has limited the ability of prior art optical moisture sensor systems to achieve a good signal balance. The optical paths shown by Muller in '931 are of unequal length. Thus, the paths would be of differing optical efficiency and could not be used to make a well-balanced system. Teder (U.S. Pat. No. 5,661,303) uses four emitters and two detectors to achieve four optical paths of equal length, however it is desirable to reduce the size and cost of the moisture sensor by using even fewer components.
The optical moisture sensor should securely engage the windshield and the optics contained therein should be optically coupled to the windshield so as to effectively eliminate the interface between the light emitters-detectors and glass surface from an optical standpoint. Purvis (U.S. Pat. No. 5,262,640) describes an intermediate adhesive interlayer for affixing the sensor housing and the optics contained therein to the windshield. The sensor housing is affixed directly to the surface of the windshield or other vehicle window by means of an intermediate interlayer disposed between the sensor housing and the interior surface of the windshield.
Vehicle manufactures desire a sensor which is already installed at the windshield manufacturer, or a sensor that is very easy to install on the vehicle production line. The windshield manufacturer ships windshields nested together such that there is very little spacing for mounting a sensor.
Schofield (U.S. Pat. No. 4,930,743) discloses the use of a bracket, such as a rear view mirror bracket, for mounting the optical moisture sensor. This approach necessitates additional support structure or the addition of silicone pieces to optically couple the moisture sensor to the windshield. A bracket mounting systems results in additional parts and increased costs.
Bendix (U.S. Pat. No. 5,278,425) and Stanton ('257) teach that a lens may be permanently affixed to the windshield such that a sensor housing may be detachably mounted on the lens. The lens may impart focal power to the beam, as shown in Bendix. Alternatively, the lens may couple the beams to the windshield through planar surfaces normal to the beam direction, as disclosed in Stanton. However, both Bendix and Stanton require a lens that is approximately as thick as the windshield. When stacking the windshields for transportation from the glass manufacturer to the vehicle assembly line, the additional space necessitated for the lens adds additional handling costs to the cost of the windshield. It is desirable to have a sensor which is attached to the windshield and is thin enough not to interfere with nesting the windshields during shipping.
Modern solar-control windshields, such as windshields sold under the trademark "EZ-KOOL" commercially available from Libbey-Owens-Ford Co., absorb the infrared rays used by many optical moisture sensors. Sensors without coupling or light gathering optics are likely to be too inefficient for use on these windshields. In German Patent No. DE 3314770 to Kohler, et. al., lenses in a coupler increase the sensed area and efficiency of a moisture sensor. Watanabe (U.S. Pat. No. 4,701,613) discloses a series of V-grooves that couples rays into and out of a windshield with an improved efficiency, however, the devices are mounted at a forty-five degree angle with respect to the glass surface because the grooves do not gather diverging light rays and focus them onto the detector. It is desirable to mount the optoelectronic components on a single planar circuit board while improving the efficiency of the optical moisture sensor for use on modern solar-control windshields.